Semiconductor lasers in common use today include edge-emitting diode lasers and vertical cavity surface emitting lasers (VCSELs). In an edge-emitting laser, a semiconductor gain medium, for example a quantum-well semiconductor structure, is formed on a surface of a semiconductor substrate. Cavity mirrors are formed or otherwise positioned on opposite sides of the substrate, perpendicular to the substrate surfaces, to form a resonant cavity which includes the gain medium. Electrical or optical pumping of the gain medium generates a laser beam which propagates in a direction along the plane of the substrate.
Edge-emitting lasers are among the most common semiconductor laser devices. Available commercially as individual units and in linear bar arrays, they are used, for example, as an optical pump source for pumping solid state lasers. High power, typically greater than a few hundred milliwatts, adaptations of edge-emitting lasers commonly operate in high order spatial modes and at multiple frequencies. This prevents their use in applications which require high power laser output in a single spatial mode and/or at a single frequency. Edge emitters also have a significant degree of astigmatism and a beam aspect ratio which is generally large, making it difficult to focus the beam to a small spot, which prevents their use in those applications which require a focused output beam. Poor beam quality in edge-emitting lasers also makes frequency doubling of the laser output using nonlinear optical materials difficult and inefficient.
In conventional VCSEL lasers, cavity mirrors are formed or otherwise positioned on opposite faces of a semiconductor gain medium grown on a semiconductor substrate. Electrical or optical pumping generates a laser beam emitted in a direction orthogonal to the plane of the substrate.
Conventional VCSELs find application in optical communications and optical interconnect systems. VCSEL lasers are characterized by generally low fundamental spatial mode TEM00 output powers, limited to about 8-10 milliwatts (mW) continuous wave (cw), and are further characterized by small fundamental spatial mode beam diameters, on the order of several micrometers (xcexcm). Larger area VCSEL emitters, with beam diameters on the order of 100 xcexcm can produce output beams having a few hundred mW of cw output power. However, operation of conventional VCSELs at high power and large diameter generally carries with it the penalty of an output beam having high-order spatial modes and multiple frequencies. In an external cavity VCSEL configuration, referred to in the art as a vertical external cavity surface emitting laser (VECSEL), an external reflector serves as the output coupler. External cavity VECSEL devices can provide higher fundamental spatial mode output power than VCSEL devices.
Previous work on external cavity vertically emitting semiconductor lasers typically resulted in low output power. The work of Sandusky and Brueck, for example, produced low output power and used optical pumping to excite the semiconductor. See J. V. Sandusky and S. R. J. Brueck, xe2x80x9cA cw external cavity surface-emitting laserxe2x80x9d, Photonics Technology Letters, vol. 8 pp. 313-315, 1996. In a study by Hadley et al., an electrically excited VCSEL in an external cavity produced 2.4 mW cw and 100 mW pulsed in a fundamental spatial mode. In this case, an emitting area up to 120 xcexcm was used. See M. A. Hadley, G. C. Wilson, K. Y. Lau and J. S. Smith, xe2x80x9cHigh single-traverse mode output from external cavity surface emitting laser diodesxe2x80x9d, Applied Phys. Letters, vol. 63, pp. 1607-1609, 1993.
For various laser applications, a beam generated by the laser is subjected to frequency conversion or frequency doubling. This is accomplished by introducing a nonlinear material, for example KTP, KTN, KNbO3, and LiNbO3 into the laser path. The frequency of a beam incident on the non-linear material is converted to a second frequency. The non-linear materials are referred to as xe2x80x9cdoubling crystalsxe2x80x9d where the property of the material is such that it serves to double the frequency of a beam traversing the crystal. Efficient frequency conversion by the material generally requires a high-intensity, single mode incident beam.
Frequency doubling of semiconductor lasers has been demonstrated in the past to varying degrees of success using a doubling crystal mounted external to an edge-emitting diode laser cavity. The output beams from edge-emitting diode lasers are usually highly divergent and have significant aspect ratios as well as some degree of astigmatism which degrades the optical field intensity and phase front from that which is ideally required for efficient frequency doubling. Experiments have been carried out in which the light from a diode laser is launched into an optical waveguide fabricated in a non-linear material in order to maintain the optical field intensity over some relatively long path length. This technique is generally complicated and uses relatively low power diode lasers which have sufficient beam quality to launch the laser light into the external waveguide.
Various techniques in the past have attempted to harness beam power to enable efficient conversion. A first technique by Gunter, P. Gunter et al. xe2x80x9cNonlinear optical crystals for optical frequency doubling with laser diodesxe2x80x9d, Proc. of SPIE, vol. 236, pages 8-18, 1980, demonstrated low efficiency frequency doubling of diode laser radiation using potassium niobate KNbO3 in a single-pass doubling configuration. In another technique, Koslovsky et al., Optics Letters 12, 1014, 1987, employed a single spatial mode, edge-emitting diode laser and KNbO3 in an external ring resonator to increase the circulating power to achieve frequency conversion. The Koslovsky configuration required frequency-locking of the single-frequency laser to the Fabry-Perot resonance of the ring cavity as well as matching the temperature of the non-linear crystal to both frequencies. This requires complicated crystal alignment and wavelength control circuitry to maintain frequency locking.
The present invention is directed to an apparatus and method for generating high power laser radiation in a single fundamental spatial mode, in a manner which overcomes the aforementioned limitations. The laser of the present invention, when configured in an external cavity, is especially amenable to frequency conversion of the output beam, as it provides beam power densities over suitable path lengths for efficient frequency conversion.
In a first embodiment of the present invention, the apparatus comprises a resonant cavity defined between first and second partial reflectors. The geometry of the resonant cavity defines a fundamental spatial or transverse cavity mode. A gain medium is disposed within the resonant cavity, and a first volume of the gain medium is adapted to be energized by an external energy source. This causes spontaneous and stimulated energy emission to propagate in the gain medium in a direction transverse to the fundamental cavity mode. The transverse emission, in turn, optically pumps a second volume of the gain medium about the first volume. When the intensity of the spontaneous emission is sufficiently high, inversion and gain are produced in the second volume. The energy within the first and second volumes is coupled into the fundamental cavity mode laser beam.
By optimizing the geometry of the cavity such that the fundamental cavity mode is coupled to both the first and second volumes, the energy of the first volume transversely-directed into the second volume, which would otherwise be wasted, is instead captured by the fundamental beam, improving the overall power efficiency of the laser. To effect this, in a preferred embodiment, the cavity mirrors are selected to match the fundamental cavity mode to the cross-sectional diameter of the second volume. In this manner, the laser energy in the fundamental spatial mode is efficiently extracted from both first and second volumes of the gain medium. Similar results apply where the output energy is in a higher order spatial mode.
In a preferred embodiment, the first volume is substantially cylindrical and of cross sectional diameter D1, and the second volume is substantially an annulus of outer cross-sectional diameter D2 and inner cross-sectional diameter D1, the first and second volumes being substantially cross-sectionally concentric.
The gain medium is preferably formed of a semiconductor material in a vertical cavity configuration. Alternatively, the gain medium may be formed of a solid state material having an active ion which has absorption in the spectral region of the gain transition. Examples of such solid state materials include Er:glass, Yb:glass, and Yb:YAG. In the case of solid state materials, pump energy would be preferably generated by optical means, for example a diode laser. In the preferred embodiment, the gain medium is formed on a semiconductor substrate at a first side and the first reflector is formed directly adjacent to the gain medium. The second reflector is positioned at a second side of the semiconductor substrate, opposite to the first. The second reflector may be monolithically grown on the semiconductor substrate, or it may be bonded to the semiconductor substrate by various techniques.
A nonlinear crystal may be placed in the optical cavity or external to the laser to change the laser output frequency. Suitable materials for nonlinear conversion include KTP, KTN, KNbO3, and LiNbO3 and periodically-poled materials such as periodically-poled LiNbO3.
A preferred embodiment of the present invention, described in detail below, is capable of generating intracavity circulating power levels in excess of 10 kW in a fundamental spatial mode for a 1 mm diameter beam. These levels are sufficient for producing harmonic conversion of the fundamental radiation in a non-linear material. As an example, frequency doubling in a semiconductor configuration using GalnAs gain media provides a fundamental wavelength of 900 nm to 1100 nm and a frequency doubled output in the blue to green wavelengths.